Tell me why not: Carbon fiber bracing.

[DMand]

So working on my old deconstructed Washburn and received a set of 3mm carbon fiber square rods just to see. They are incredibly stiff and light. Has anyone tried lattice or Xbracing exclusively with thin carbon rod? A mix of spruce and CF? Would the CF rod be so rigid as to kill the sound?

Thanks! Dave H

[eightmoremiles]

I have a 1975 Gibson F5 which has been redone-top and back recarved to Loar specs-and refinished with thin varnish rather than thick lacquer. The tone bars were short and thick, and have been replaced with thinner, more gracile tapered tonebars, each a sandwich of spruce on the sides and carbon fiber in the middle. The net product is much louder than the original, with an almost startlingly quick response and bright tone.

[DMand]

Then why not use CF rods directly glued to the soundboard and dispense with spruce?

[sunburst]

Nasty dust, difficult to glue. Other than that, the main thing is getting a wide enough brace to have enough surface at the glue joint for a good joint, while keeping the brace light in weight.
Look at it this way:
if a spruce brace can be carved to a good, functional stiffness and mass at a reasonable size, how much smaller would a CF brace have to be to match that stiffness and mass? If you want a different stiffness and mass, CF may well be the answer, but the dust is still nasty.

[MikeEdgerton]

The late Rick Turner used carbon fiber sandwiched bracing on at least one of the guitars they manufactured and also used CF rods inside the instruments. I'm sure there's more out there on the web someplace.

https://acousticguitar.com/rick-turn...-model-guitar/

[Graham McDonald]

There is a modern school of classical guitar builders who use a balsa/carbonfiber lattice for soundboard bracing. The CF is top and bottom of the balsa making it like an I beam, rather than sandwiched vertically between two pieces of spruce, which does add stiffness but also adds weight. With the edge of the CT exposed where the brace has to be carved or shaped, it is very hard to do with edged tools (they will not have an edge very quickly) and they would have to be shaped by something like a little sanding drum on a Dremel, which puts lots of nasty dust into the air. I have tried a couple of carved mandolins with a small CF/balsa lattice in the middle, but so far just too stiff and they have sounded horrid.

[keith.rogers]

I think a laminated/sandwiched brace would be easier to work with, and on a flat-top instrument, probably avoid the dust problem. Just using CF seems like it would mean resorting to something like epoxy for fixing it to the top, which would probably not make some folks happy, but it might be an interesting experiment if you've got the thing apart and the material handy. (The rods would seem to be problematic if that's the shape you're working with.)

[sunburst]

While there is nothing fundamentally wrong with the idea of solid CF braces, they would need to be correctly sized to get them into the range of mass and stiffness that works for the sound we want. That can be done with spruce braces more easily. The one potential advantage of CF (that I can think of) is the material's stability under constant strain, so string tension and bridge pressure/torque may not distort the shape of the brace over time. Distortion of brace shape over time is generally not a big problem in well designed, well built, and well cared for instruments, but it does happen sometimes. Since part of the shape change involves the top itself, how much control of the situation could we expect from CF bracing? (I don't know.) and how reliable would brace glue joints be? (I don't know.)

[rcc56]

All I know about it is that a friend of mine who has built a few instruments built a guitar with laminated CF/spruce bracings.
A couple of the braces eventually split, and he said he would not build another one.

Also, I don't know how one could voice and trim the braces after they had been installed, since CF eats up cutting tools. You're not going to be able to chisel or plane it.

[peter.coombe]

I use carbon fiber tow on the tops of the main structural braces on my flat top mandolins. It is glued to the brace with Epoxy after the braces have been shaped. The braces are feathered at each end to make an arch. The downwards force of the bridge pulls on the carbon fiber, and since carbon fiber is strongest when pulled it makes a very strong structure and the top will not sink like most flat top mandolins do. In practice it works very well, the tops do not sink, and the instrument stays in tune remarkably well, and they sound damn near as good as my carved top mandolins, some are even better. However, once the carbon fiber is installed I can't make any adjustments to those braces, but I know from experience how the carbon fiber changes the stiffness of the top so I adjust the braces and measure before gluing the carbon fiber on. At first I used an X brace similar to a steel string guitar, but the latest development is to use a bracing pattern similar to a classical guitar. It uses more carbon fiber tow and the braces are quite small so the tops are about 10% lighter. This bracing pattern would not work without the carbon fiber, the top would sink. The carbon fiber allows me to make the braces significantly smaller, thus saving weight on the top. The weight of the bracing on a flat top mandolin is about 20% the weight of the top, whereas on a carved top it is less than 5% so on carved top mandolins you won't save much weight, but you will gain some stability. Using a carbon fiber laminate or solid carbon fiber causes more problems than it is worth. If you want to learn more about use of carbon fiber tow, read the guitar design and build books by Australian luthiers Trevor Gore and Gerard Gilet. Highly recommended and relevant since mandolins do vibrate like guitars.

[DMand]

Really interesting! Do you have any photos of a braced top? Thanks! Dave

[Jeff Mando]

I guess the reason for asking this question is the idea appeals to you. For that I say, why not?, go for it.

I thought about the Garrison acoustic guitar from Canada with its fiberglass bracing. They have their fans who swear by them. Gibson bought the company and promptly closed it -- so maybe they felt it was a threat? I don't know. I do know most people have never heard of a Garrison.

For me, the tradition of wooden instruments works fine and makes sense to me. For others, experimental and alternative material is appealing. Most people don't know that Les Paul was a fan of (the then new) plastic guitars of the late 40's/early 50's, working with the Maccaferri company. If Maccaferri could have matched
Gibson's offer, the Les Paul guitar might have been made of plastic, FWIW.

Will carbon fiber bracing sound better? I doubt it. Is it stronger than wood? Maybe, but then again, how strong does it need to be? The basic thinking is that you want the top and bracing as thin as you can get away with for the best sound -- stopping (just) short of the thing imploding by the string tension.

Good luck with whatever you decide.

[j. condino]

In the mid 1990s, I made a series of guitars with laminated carbon fiber braces- most were sitka spruce with .020-.030" carbon fiber center beams, glued to redwood tops from the Lucky Strike tree. The overall result was braces that were much lower in mass as well as much stiffer- too stiff. When the weather changed severely about a year later, the braces were so stiff they literally ripped off the top. As for the voice, they never sounded quite as well as I hoped. Combine that with the dust and dulling of tools, et cetera, and I dropped the process.

My overall conclusions were that I like working with wood and I have been accustomed to the voice of wooden guitars. My modest experience with building guitars at that time was the problem, not the materials choice. I had gotten caught up in all of the hype and tried to bypass learning how to build a great traditional guitar by using a fancy material, rather than learning how to build a good guitar. These days, I obsess about the small details with traditional spruces and I am amazed at what an incredible structural material they are.

[Jim Garber]

Carlo Mazzacarra has a bowlback just listed in the Classifieds with spruce and CF braces.

[Richard500]

Haven’t made any mandolins, but the basic issue in combining dissimilar materials is that bonding two things that have different thermal coefficients of expansion and dimensional changes with ambient humidity is either going to rip the bond or deform the structure. It is a good recipe for making, say, a bi-metal thermometer, and some optical structures, but given that wood tends to easily separate along the grain, one might try to avoid stressing it.
Use along the grain, as in a neck reinforcement is much more reasonable.
By the way, following the recent discussion of adding CF arrow shafts to mandolins, I looked up sizes, prices for these and other CF forms. Definitely inexpensive, even in single units. No need to find urban archery stores.

[Bob Clark]

I haven't read through all the posts, so forgive me if I am repeating someone else.

Over the course of many decades, Rolfe Gerhardt was quite innovative in building and perfecting Phoenix mandolins. His mandolins were things of great beauty and voice. He experimented with, and adopted, carbon fiber in his braces early on in the evolution of this material into stringed instruments. As I recall, he used a carbon fiber/spruce laminate to keep his braces light but stiff. I had one such instrument and it was very light, with a truly beautiful voice. How much carbon fiber bracing contributed to that, I do not know.

[sunburst]

...the basic issue in combining dissimilar materials is that bonding two things that have different thermal coefficients of expansion and dimensional changes with ambient humidity is either going to rip the bond or deform the structure...

Because wood has different amounts of movement in different directions (more movement tangentially, less movement radially and essentially none longitudinally) wood workers are taught to avoid cross-grain joints, to allow for wood movement, and otherwise accept that wood will move and make allowances for it. That is why, when viewing the inside of a guitar for the first time, with it's cross grain glue joints and generally "poor" structural design, the reaction often looks like...

[Steve Sorensen]

Why go half way?



Carbon fiber is just a different material. Depending on how it is used, it is 4 to 8 times stiffer than maple or spruce, and twice as heavy as sugar maple. Of course you can add lots of other material in the mix ... kevlar, nylon, steel, microspheres, wood flour, wood elements, paper. Each element has individual traits and more complex traits when mixed together.

The real question is not, "Why not?" but rather "Will it make a better instrument that players want to play?"

Steve

[mandocello8]

my gibson a-4 mandolin was rebraced (and carbon fiber rod added to the neck). Having just have seen fancy space age carbon fiber bracing at a guitar show, I asked the luthier about using something like that. his answer: carbon fiber cannot be glued by hide glue, only hide glue is to be used for work on old gibsons. fair enough.

[DMand]

So, I am leaning toward trying a floating diagonal lattice CF with 6mm square rods that do not go all the way to the edge. Anchored with CF cyanoacrylate or epoxy. I’m going to reinforce the oval soundhole with a ring of CF and some CF rods vertically to the neck joint. I may try some rods anchored diagonally to the neck and tail posts and a few wood blocks midway on the sides to stiffen the sides. The idea is to stiffen the sides, and create a top that is stiff but can pump like a speaker since the braces don’t go all the way to the kerfing. Will keep everyone posted and post pics when I get all the warps and cracks fixed on the top. If it’s a disaster y’all be the first to know. Dave

[DMand]

Maybe 3mm if I can cut the notches

[Dave Cohen]

You can save yourself a lot of time and trips into blind alleys by doing some reading first. I'll second Peter Coombe's recommendation of the Gore & Gilet book, and add a few more recommendations. One is my chapter, as well as the first few chapters, in "The Science of String Instruments" (Springer, 2010), edited by the late Thomas D. Rossing. Another highly regarded work for understanding how plucked string instruments work is the Dissertation by Howard Wright, "The Acoustics and Psychoacoustics of the Guitar" (1996), done under the supervision of Bernard Richardson at the University of Wales, College of Cardiff. There are numerous other pertinent works as well. You will find some of them referenced in the above-mentioned works.

Finally, a lattice of 6 mm square CF composite rods will be extraordinarily stiff. An instrument with bracing that stiff will have about as much acoustic response as a solid-body electric guitar body, i.e., no response at all. It is not enough for the edges of a plate to vibrate; entire top plates (AND back plates) have to vibrate in their normal modes of motion. A plate braced such as you describe would be so stiff that if it would not vibrate except at frequencies far too high to be of any use in moving air in the acoustic time domain.

[peter.coombe]

Yep, 6 x 6 mm CF is real stiff and a lattice of it would be extraordinarily stiff, so don't do it. CF tow is supplied like a piece of string wound around a bobbin. It is not stiff until it is embedded into a resin, and the combination is called a composite material. Here is a picture of what I am using on my flat top mandolins now. Have only made 2 so far plus a mandola. The mandolins were an Army Navy style and one of my own design I call a Classical Flattop. The mandola is one of the best sounding I have made, flat top or carved top, and the mandolins are by far the best sounding flat top mandolins so far. I can hardly wait to try it on an octave mandolin. The braces are only 5mm wide and max 9mm (10mm on the mandola) high (main cross brace and center brace under the bridge). The braces taper down in height as you go out from the center. Now, the bracing of the top is only part of the story. How you make the back is just as important. Mine are made as a "dead back" system (see Gore and Gilet book), so think heavy, high internal damping woods, and hefty braces. Maple is not a particularly good wood for this system, but I have access to many species of heavy native woods. They are not actually "dead" backs, it is not black and white, but the contribution of the back to the sound is much reduced. The top and back are matched according to what is recommended by Gore and Gilet, they got it right. It is somewhat counter intuitive, but the heavier backs make a louder mandolin (and guitar). That is because the top moves more. Think of a see saw with one man heavier than the other. If the see saw is balanced by moving the fulcrum, the lighter man will move more. That is exactly what happens. The main top mode node (the fulcrum) moves out to the rim of the body. With a light back, the node is about 1inch in from the rim so the rims are moving and wasting energy not producing sound. A heavy back has a similar effect as adding mass to the sides of guitars as is described in the Gore and Gilet books. If the back is too stiff I can adjust it by filing down the center brace on back through the end pin hole. I measure the frequency of the main top and back modes with the strings on, but you can also determine if you need to reduce the stiffness by sticking some blue tack to the back and listening to the sound. If the sound improves with the blue tack then the back is too stiff. Blue tack adds mass and will lower the main back mode frequency. To get the recommended frequency interval, the back does need to be really really stiff and I can get it into the ball park from the frequency of the free plate modes, but the relationship to the assembled instrument is not exact, there is a lot of scatter if you plot the frequencies so I don't get it spot on every time.

Note that as the mass of the top is reduced and stiffness is maintained, there will be more higher frequencies so the instrument will sound brighter and louder. This is simple physics and is explained in the Gore and Gilet design book. So, a warmer sounding back wood helps to make a better balanced sounding mandolin. Jarrah and Red Gum work really well, as does Macassar Ebony. I play a Macassar Ebony mandolin in a band and it has a massive dynamic range and can be heard. Note that these mandolins are all oval hole mandolins and oval hole mandolins usually get drowned by loud fiddles etc. This one does not.

[peter.coombe]

Just a small point, but I have let the cat out of the bag a bit with the above post. I was going to write it all up and send it to the GAL, but that is low priority at the moment I am far too busy filling orders, mostly guitars at the moment. It represents over 10 years of work and around 50 mandolins to get where I am now. So if you implement any of the above please acknowledge the source, i.e. me. We all stand on the shoulders of others and I stand on the shoulders of my friend Graham Caldersmith who is no longer with us, Cohan and Rossing, Gore and Gilet, plus others that have all been acknowledged in the articles that are available on my web site. Graham Caldersmith's work on guitar acoustics was pioneering stuff without which the Gore and Gilet books would not exist. Without Cohen and Rossing showing that mandolins vibrate like guitars I would not have made the connection to the guitar research. IMHO the Gore and Gilet books are by far the best books ever written on guitar construction mainly because they use sound engineering principles that is based on the guitar research and it is properly acknowledged so you can follow it up, not like some other publications that state "facts" that are not supported by the physics. I would like to add that if you get the book "The Science of String Instruments" (Springer, 2010) as recommended by Dr Cohen then it is also worth reading the chapter written by Graham Caldersmith.

[DMand]

Peter, thank you and I am going to order that book! I’m thinking of stiffening the back as well now as well as adding vertical rods midway up to connect the tail and head block, maybe a set of horizontal rods placed on new blocks horizontally. The English luthier NK Forster has been making his guitar sides thicker and he tells me that this has made a big difference in sound volume. So Im thinking that more rigid sides will let the floating lattice soundboard vibrate. I ordered some 1 mm rod as well and if it’s stiff, I may try a different bracing arrangement. Dave